\(2cos(kz-\omega t) \)
this equation represents upward moving waves. Simply at constant phase\(kz-wt=constant\)
by differentiating it, we get\(kdz=wdt\)
, so\(w=dz/dt=w/k\)
, or you might thing this way that at constant phase\(kz-wt=constnat\)
, then if time increase, then z must also increases which means upward motion.\(2cos(kz-\omega t) exp(-z/H) \)